new version of burn-down chart (new version: beta): showing the days where tickets were closed and the effort that is left

parameters:

width: width of the chart as pixel, e.g., width=800; default: 800

height: height of the chart as pixel, e.g., width=400; default: 350

first (mandatory): first day of the chart as DIN date, e.g., first=2010-11-24

last (mandatory): last day of the chart as DIN date, e.g., last=2011-01-02

segment: time segment in which the tickets are aggregated, e.g., segment=week; default: day, allowed: day,week,twoweek,month,year

field: multiply the ticket change with the weight for example with story points or the estimated time (a number field), e.g., field=storypoints; default: status (ticket status has weight 1)

finishstatus: depending on the actual Trac workflow the status for closing a ticket might change, then it can be configured here, e.g., finishstatus=closed|reviewed; default: closed|in_QA (as needed for the standard Trac workflow on ​http://trac.edgewall.org/wiki/WorkFlow/Examples)

Because this is a rather special macro I decided to include the functionality
in Stats wiki macro by using keyed parameters. Added bonus is an optionalprefix key to only include pages with matching names.

If it seens preferable to retain the original macro as a short-hand, this could
still be done by adding it later on and treat it as an alias internally.

Use of user_time (from Trac 0.13) defeated the value of the compat function.
The syntax for inheritance of USER_VIEW by ACCTMGR_USER_ADMIN is corrected,
and finally ACCTMGR_USER_ADMIN now inherits EMAIL_VIEW from Trac core too,
because setting user properties without seeing them by default felt wrong.

USER_VIEW permission is required, where anonymous users could learn about
sensitive information like existing accounts/users. This permission
shouldn't be granted lightly in publicly available Trac applications,
because it has the potential to encourage efficient brute-force attacks
without the need to guess existing accounts.

Now the time stamps are combined with a relative time interval hint (tool-tip).
This is an enhancement to the user admin panel too.

Support for bleeding-edge user configurable time in Trac 0.13 is accompanied
here by a fallback for Trac 0.11 and 0.12, that looks great and is worth a lot
of the effort put into this rather complicated fallback code.

The user query link is currently not implemented similar to UserStatsMacro,
but users with `ACCTMGR_USER_ADMIN permission will see links to user details
instead, like in recent version of the user admin panel.